BibFormat: improve plot file identification

* Identify (and hide) "Plots" on the basis of their doctype,
  not based on file extension or caption. (closes #1514)

* Some CERN-specific handling of the files for the display.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Reviewed-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Merge pull request #18 from egabancho/1514-master-bibformat-hide-plots-on-doctype

BibFormat: improve plot file identification